For safe aircraft operation, there is a need to provide an inert or combustibly inert material, such as Nitrogen Enriched Air (NEA) for filling, void space in the fuel tanks in order to maintain the required amount of fuel pressure in the fuel tanks while concurrently minimizing all chances of fire within the fuel tanks. One example for providing such an inert gas is to utilize an On-Board Inert Gas Generating System (OBIGGS) wherein combustibly inert gas is produced by processing engine bleed air or aircraft cabin air with an inert gas generator, which is a gas or Air Separation Module (ASM), preferably based on a selectively Permeable Membrane (PM) technology.
The ASM consists of hollow fiber bundles packaged in a cylindrical shell, with an inlet and an outlet, generally at opposite ends, and a shell side vent port. Commercially available polymeric fiber gas separation membrane technology is employed. Briefly, when pressurized air passes through the hollow fibers, via the ASM inlet, oxygen is separated out by diffusion through the fiber walls and exits through the shell-side vent port. The remaining NEA flows out of the ASM via the outlet port and is distributed to the ullages of the aircraft fuel tanks for the purpose of flammability reduction or irritating of the fuel tanks. The diffusion fibers operate most efficiently, in terms of selectivity of oxygen over nitrogen (ratio of their respective permeability) at a specific temperature, which is higher than ambient.
In most commercial aircraft, the compressed air for NEA generation originates from either engine bleed air or from cabin air, wherein, in the latter case, a separate motor driven OBIGGS radial flow centrifugal compressor compresses the cabin air to a higher pressure. In both cases, the hot compressed air is cooled by an OBIGGS heat exchanger to a temperature optimal for ASM fiber performance. A consistent remaining problem is that during aircraft cruise condition, the amount of OBIGGS airflow through the ASM is very low, such that the air traveling through the fibers can easily be quenched by the relatively cold ASM shell, at a high altitude ambient environment. Therefore, it is beneficial, from an air separation performance standpoint, to keep the ASM shell warm by utilizing external means.
In an electrically pressurized OBIGGS system, using air compressed by a motor-driven radial flow centrifugal compressor, this compressor has to operate within a wide range of flow rates and pressure ratios in order to satisfy the variable flow demands under various flight conditions. A motor controller controls the compressor to operate at different speeds. In order to prevent the compressor from surging at low flow demand conditions, such as during cruise operation, the compressor design flow rate is deliberately increased over the ASM flow demand. The excess compressed air has been disposed of by exhausting same into the OBIGGS bay of the aircraft via a surge relief valve at the heat exchanger outlet, located upstream of the ASM. Thus, the energy required to compress this excess amount of air is essentially wasted.
The patent literature includes a rather large number of systems and methods for processing a supply gas having two or more components for obtaining a product gas enriched in one of such components and include: U.S. Pat. No. 3,922,149 to Ruder et al.: U.S. Pat. No. 4,556,180 to Manatt: U.S. Pat. No. 4,681,602 to Glenn et al; U.S. Pat. No. 4,944,776 to Keyser et al.; U.S. Pat. No. 5,470,379 to Garrett; U.S. Pat. No. 6,458,190 B2 to Dolle et al.; U.S. Pat. No. 6,491,739 B1 to Crome et al. None of the cited references excess pressurized air, available from a radial flow centrifugal compressor to keep the ASM warm when the aircraft cruises at high altitude and low ambient temperature. The present invention also optimizes the performance of the ASM by maintaining the temperature of the ASM at the optimal operating temperature, as well as to utilize the energy of this compressor, which is otherwise wasted, if not used. As will become clear hereinafter, the energy of the excess air is also additionally utilized for driving a turbofan for conditioning the ASM inlet air during certain operating conditions.